The present invention generally pertains to battery testers, and more particularly, relates to on-label thermochromic battery testers.
Batteries are often stored before being used. Batteries are typically stored by retailers before being sold. After purchase by a consumer, such batteries are again typically stored for some period of time prior to use. If the period of storage is significant, batteries may self-discharge. Therefore, it is desirable to utilize a battery tester to determine if a battery has sufficient charge to operate a desired device.
It is also desirable, on frequent occasions, to determine the remaining life of batteries which are in use. Many "good" batteries are discarded simply because the user cannot recall how long they have been used in a particular device, i.e., a camera, tape deck, etc. For similar reasons, batteries often reach a useless or near useless state of discharge when no replacements are readily available. Separate or stand-alone battery testers are known which indicate remaining battery power. However, such testers are easily misplaced and cumbersome to use.
Battery testers have been described that are included in a label secured to a battery. One type of on-label battery tester is known as a "thermochromic battery tester." Thermochromic battery testers typically include a conductive element that is selectively connected between opposite terminals of the battery. The conductive element includes a switch pad at one or both ends that is pressed by the user to connect the conductive element across the terminals of the battery. When the conductive element is connected between the battery terminals, it generates heat as a function of its resistivity and the current flowing from the battery. The level of current produced by the battery is one indicator of remaining battery capacity. Thermochromic testers further include a thermochromic layer, which changes its color or visual appearance as a function of the heat generated by the conductive element. By changing the visual appearance of the thermochromic layer, a thermochromic on-label battery tester may provide an indication of the discharge level of the battery. For example, a thermochromic material that changes between opaque and transparent states may be utilized to expose indicia underlying the thermochromic layer indicating that the battery is still "good" when a sufficient level of current is output from the battery.
To produce such thermochromic battery testers in the simplest manner and at the lowest cost, the conductive heating element is printed at a single printing station using a single conductive ink material. A suitable material that may be readily printed on a substrate and that exhibits suitable heat-generating properties in response to the current from the battery, is silver ink. The silver ink is typically printed so that the resistivity of the ink film is the same in all portions of the circuit. Although the thermal flux can be focused by making the circuit narrower and the resistance higher where a high flux is required, the entire circuit acts as a heater. As a result, a low resistance circuit which draws high current is required to produce sufficient thermal flux in the display section. Because silver ink is relatively expensive, the conductive heating element thus constitutes a significant portion of the total cost of providing a battery tester on a battery label.
Another problem associated with conventional thermochromic on-label battery testers is that the contact switches that a user is required to press to activate the tester may become relatively hot due to the high current levels flowing through the conductive element. Although the heat generated at the switch contacts is not hot enough to burn the user's fingers, this heat nevertheless may create an unpleasant sensation for the user.